Distillation of butane containing high boiling contaminants



Patented Oct. 19, g

DISTILLATION 0F BUTANE CONTAINXN HIGH BOILING CONTAMI'NANTS Glcnn'W. Wilson, Jr., Goose Creek, Tex., assignor to Standard Oil Development Company, a corporation of Delaware Application July 29, 1944, Serial No. 547,251

3 Claims. (Cl. 202-40) The present invention is directed to a method 7 for preparing a butane feed stock. More specli'lcally, it is directed to the preparation ofa normal butane feed stock suitable as a charging material in isomerization processes.

It is necessary for the composition of the feed stock prepared for isomerization reactions to be carefully controlled. The yield, rate of reaction and catalyst life in isomerization processes are adversely effected by the inclusion in the feed stock of contaminants having a higher boiling range than the desired parafllnic hydrocarbons. As a specific example, in the isomerization in the vapor phase of normal butane the inclusion of pentane and heavier components, such as seal'oil, in the stock charged to the lsomerization process is extremely detrimental to the life of the catalyst and in addition causes the formation of undetillaton tower a composition including normal and isobutanes, as well as a small portion of pentanes and seal oil. In the distillation tower, the

major portion of the isobutane is removed as overhead while a bottoms fraction is withdrawn as the fraction to be used as the feed to the i omer'zation unit. ,Thisbottoms fractions is contaminated by, substantially all of the pentanes charged to the distillation tower and by the seal oil charged to the tower, as well as additional seal oil picked up from the pump circulating the reflux to t e tower.

It is an object of the present invention to prepare a hydrocarbon'stream suitable for use as the charging stock to an is'omerization unit, comprising a major portion of normal butane and includ ng substantially no pentanes and heavier hydrocarbons. such as seal oil.-

Other objects and advantages of the present invention may be seen from a reading of the following description taken with the drawing in wh-cb the sole figure is in the form of a diagrammate flow sheet. v

A mixed feed is discharged byline ll into dist'llation column i2. It will be understood that st-Nation column i2 may be a convent onal bub le tower provided with a large number of trays to aid in the satisfactory scparfation of fractfons having different boiling points. Vaporization takes place within the column and a vaporous fraction is removedas overhead through line ll, is condensed in condenser l4. and is discharged from the condenser by line I! into storage drum IS. The liquid accumulating in drum it may be withdrawn through line H and pump It. It is desirable that the major portion oi the condensate be returned to the tower to serve as reflux therein and this portion of the stream may be returned by means of pump l8.and line I! to the top of the tower. A minor portion of the condensate may be removed either continuously or r intermittently as product by outlet 20.

Heat from an external source may be conveniently supplied to the material undergoing rectification in tower H by means of reboiler 2|. The reboiler shown is of the conventional type, having steam inlet 22- and steam outlet 23. It will 'be understood that the steam is passed through tubes, not shown, in the reboiler and supplies heat to the reboiler byindirect heat exchange. Reboiler 2| is provided with a partition 24 which is arranged to form a weir and liquid may be injected into a lower portion of the reboiler on one side of the weir by inlet lines 25 and 26 and may be discharged from the lower portion of the reboiler on the other side of the weir through line 21. Vapors may be returned from the reboiler to the distillation column via line 28.

It will be evident that the extraneous heat supplied to column l2 for refluxing is added thereto by the bottoms fraction flowing from the bottom of the tower through line 25 to the heating sec- 2| than is vaporized-therein so that a minor portion of the ingredients charged to the reboiler fiow over the upper edge of weir 24 as a stream and is withdrawn continuously through line 21.

' The major portion of the stream withdrawn through line 21 will be normal butane.

It is advantageous to discard this stream because of the substantial amounts of heavier hydrocarbons,

be withdrawn from the lower portion of-tower i2 via line 29.

The following is given as an example -oi suitable operating conditions in the practice of the present invention with the composition 01' the stream ledv into the system and the stream removed therefrom. In a tower containing 40 plates, a feed stream is discharged above the 24th plate at a temperature of 145 F. The volumes of com- The vapors are removed as overhead through line it at the rate of 78,000 gallons per hour and 72,000 gallons per hour are returned as reflux while the remainder, 6,000 gallons per hour, is withdrawn as product. The pump l8 used to return the reflux adds 200 pounds of seal oil per day to the refluxof'the tower of which some 185 pounds are returned to the tower. The composition of that portion of the overhead withdrawn as product is as follows:

Table III Gals/hour 1C4 1 4,800 NC4 1,200

Total 6,000

Bottoms are withdrawn from the tower .to the steam reboiler at the rate of 89,000 gallons per hour while vapors are returned 'from the reboiler to the column at the rate of 90,000 gallons per hour. Bottoms withdrawn from reboiler 2i via line 21 at the rate of 3,000 gallons per hour have the following composition:

Table IV Gals/hour 1C4 50 N04 2,941 105 9 Total 3,000 Seal oil, 293 lbs/day.

The hydrocarbon stream withdrawn via line 20 as the feed for a butane isomerization unit at the rate of 10,000 gallons per hour, has the following composition:

Table V Gals/hour .IC4 210 N64 9,773 105 17 Total 10,000 Seal oil, 22 lbs/day.

With this procedure the top of the tower is maintained at a temperature of 135 E, the bottom of the tower at a temperature of 164 1". and the condensate accumulating in drum II has a temperature of 128 F.

It will be seen that the stream removed by line 29 has present therein approximately 2 per cent of -isobutane and negligible amounts of isopenponents added per hour are shown in the following table: 7

Table I Gals/hour 1C4 4,980 N04"; 10,002 1C5 18 Total 15,000 Seal oil, lbs/day.

Feed is added to reboiler 2i via line 28 as indicated in the following ,table:

Table II Gals/hour 164 s 80 N04- 3,912 105 5 Total 4,000 7 Seal oil, 100 lbs/day.

nes and heavier hydrocarbons, such as seal oil, and is a satisfactory feed stock for use in the isomerization of normal butane. It will be understood that the above example is given only by way of illustration and not by way of limitation. It will be evident to the skilled worker that the rate of operation, the composition oi the streams and the temperatures employed may .be varied over a substantial rangewithout departing from the scope of the invention. Y

' Having fully described the what I desire-to claim is:

1. A continuous method for distilling a primary feed mixture including at least a first component, a higher boiling second component, and a com taminating amount of a component boiling substantially higher than said second component comprising the steps of continuously introducing a stream 01' said primary feed mixture into a medial portion of a fractional distillation column, maintaining the distillation column under temperatureand-pressure conditions to cause removal of predominantly said first component substantially free of the contaminating component as a vapor from the top of column, continuously present invention.

r. condensing said vapor to form a condensate, continuously returning at least a portion 0! said condensate as reflux'to an'upper part of said column while recovering another portion as distillate product, continuously removing from the bottom of the column a first and a second liquid stream, said first streamv being of greater volume than the second and each stream comprising predominantly said second component regulating the relative quantities of said first and second streams so that said second stream contains less than 10% by weight of the total high boiling components in the system, continuously recovering said second stream as liquid product, continuously introducing said first stream into a heated evaporation zone, supplementing the quantity of second component producible from said primary feed mixture by continuously introducing into the evaporation zone a second feed stream from a sourceexternal to said distillation column and diiierent from said primary feed mixture, said second feed comprising predominantly said second component and contaminating amounts of higher boiling components substantially in excess of the amount in said second stream, continuously vaporizing a major portion of the second component in the evaporation zone, continuously returning the resultant vapors to a section 01' the distillation column above the point of withdrawal of said first liquid stream but below said medial portion, and continuously removing from the evaporation zone a liquid stream including approximately 90% by weight of the total high boiling contaminants introduced into the distillaportion oi higher boiling impurities to a medial portion of a distillation column, continuously maintaining the distillation column under temperature and pressure conditions to cause removal of a major portion of the isobutane charged to the column as overhead in vapor form, continuously condensing said vapor, continuously increasing the pressure on the resultant condensate and adding substantial amounts or higher boiling impurities thereto while increasing the pressure thereof, continuously removing a small portion of the condensate as product and returning a major portion of the contaminated condensate to the distillation column as reflux, continuously removing from the bottom of the column a first and a second liquid stream, said first stream be.- ing of greater volume than the second and each stream comprising predominantly normal butane, regulating the relative quantities of said first and second streams so that said second stream contains less than 10% by weight of the total high boiling components in the system continuously introducing said first liquid stream into an evapporation zone, supplementing the quantity of normal butane producible from the primary feed mixture by continuously introducing into the evaporation zone a second feed stream derived from a source other than said distillation column and the primary feed mixture, said second feed stream comprising predominantly normal butane contaminated with high boiling impurities substantially in excess or the amount in said second stream, continuously vaporizing a major portion of the normal butane in the evaporation zone and returning the resultant vapors to a lower portion of the column, continuously recovering from the bottom of the distillation column said second stream comprising a major portion of normal butane substantially free from higher boiling impurities and suitable as'feed for an isomerization step, and continuously withdrawing from the evaporation zone a liquid stream comprising substantial amounts of normal butane and approximately 90% by weight of the higher boiling impurities added to the distillation column and to the evaporation zone.

3. A method in accordance with claim 2 in which the upper portion of the distillation column is maintained at a temperature of the order of 135 F., the lower portion of the distillation column I is maintained at a temperature of the order of 164' F. and in which the contaminants higher boiling than normal butane are present in the withdrawn bottoms fraction and in which the mixture is charged to the distillation zone at the rate of 15,000 gallons per hour, the fraction 'is added to the evaporation zone at the rate of 4,000 gallons per hour and in which the bottoms fraction is withdrawn as a product at the rate of 10,000 gallons per hour.

GLENN W. WILSON, Jr.

REFERENCES CITED The following references are of record in the 2,335,162 Shiras NOV. 23, 1943 

